When I start, I just dont know who to contact. I cant change to the frequence wich you can see in the chart, because there is no atc.
The ATIS will tell you when the airborne frequency has been revised to something else.
Thx. one other question. what is the trans alt?
Transition altitude
In this case you are looking at an airport that doesn’t use an autohandoff, so ATC will tell you which frequency to tune to after departure. And there is simply nobody covering EDDM right now, so obviously there is also no ATIS.
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I haven’t used it personally, but VATGlasses https://vatglasses.uk/ might be a help here. If I recall correctly, it supports mainly Europe. It attempts to depict airspace three-dimensionally better than the legacy tools (which typically depict most airspace simply as a 2D circle).
In general:
If you’re outside controlled airspace, use Unicom.
If you’re uncertain if a particular controller is controlling an airport/airspace you’re in, call them and ask – you should be met with a friendly reply. (Though in honesty, something wildly egregious may earn you a slightly snarky reply – like calling Dublin Approach asking if they can provide you service when you’re on the ground in Rome, as an example… 
)
If a controller pings you (“contact me” message), reply to them on their frequency.
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Though, frankly: don’t use map tools to try and figure out what the current airborne frequency for an autohandoff is.
Due to crosscoupling of these frequencies being forbidden for some reason (while it is allowed in situations where it makes much less sense, but alas), the charted frequency will not always be in use. VATSIM unfortunately also doesn’t ensure that there is some kind of standard of how to deal with these situations, so every vACC has their own approach to this issue, but none of them involve “we expect the pilot to look at some map and figure out the frequency on their own”.
In Germany, which is where OP’s screenshots are from, the ATIS will include the currently applicable airborne frequency, or - in the rare case that the airport doesn’t have an ATIS - will be given by ATC some time before departure. But other vACCs have other ways of addressing this issue. In some, it is standard procedure to always tell pilots what airborne frequency is in use with the takeoff clearance; others expect pilots to follow the charted procedure unless something different has been stated (which will only be the case if the charted frequency is not currently in use).
Map tools are really only relevant when trying to figure out if someone is providing topdown service for a given airport/airspace or when entering a sector coming from unicom, not for figuring out which controller to switch to and when while already talking to another controller.
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Give VSR a go vsrsoftware.com
It’s a toolbar app designed to help you see who is online, who to contact next etc.
Vatglasses covers the world simply put your airport icao code in the search feature and it will tell you exactly who to contact if the airport is currently under atc Also in the US they are trialling CTAF so rather than UNICOM at an airport use https://my.vatsim.net/pilots/aip search for your airport and use the CTAF frequency till you are in the air, if GND is on but no tower GND may pass you to CTAF
Transition Altitude where you switch to standard presure 1013 hp